1. Field of the Invention
The present invention relates to a method of producing a semiconductor wafer suitable for producing semiconductor devices.
2. Related Art
In a semiconductor device, if a region in a wafer where devices are fabricated (generally an obverse surface) is contaminated with metallic impurities, particularly transition metals, the characteristics of the device are deteriorated substantially in that the minority carrier lifetime is short, and that the leakage current of a p-n junction is high. Accordingly, it is essential to prevent the metallic contamination of the region in a wafer where devices are fabricated (hereafter referred to as the device region) as practically as is possible.
As means for preventing the above-described contamination, various types of gettering have conventionally been adopted (S. M. Sze: "VLSI Technology" McGRAW HILL, 1983, .sctn.1.5.1).
If a wafer contaminated with metals is subjected to heat treatment, metallic atoms in the wafer move due to thermal diffusion. If there is a sink within the wafer with respect to the metals, the metallic atoms are trapped and absorbed thereby and are fixed thereat.
Accordingly, if sinks are created in advance at a high density within the areas excluding the device region, even if the wafer is contaminated in the subsequent device fabricating process, the extent of contamination of the device region can be reduced by heat treatment.
Such a technique is called gettering, and can be largely classified into the following two categories:
(1) Extrinsic Gettering (EG)
This is a method whereby the back surface of the wafer is intentionally damaged through mechanical abrasion by means of sand blasting, lapping or the like or through the application of a focused laser beam.
(2) Intrinsic Gettering (IG)
This is a method whereby microdefects are caused to occur at a high density in the interior of the wafer, while defect-forming atoms (oxygen, for example) are removed from the surface of the wafer by evaporation, thereby forming a denuded zone (DZ).
The backside damage (BD) and the microdefects respectively constitute as sinks for the metallic impurities, and their effectiveness has already been confirmed (ibid.).
In the method based on sand blasting, abrasives composed of compounds including elements that are common with those of the wafer are generally used to prevent contamination. For instance, in cases where the wafer is formed of silicon, abrasives such as quartz and carborandum are used. An examination of their configurations revealed that they displayed configurations having pointed corners, as shown in FIG. 6.
Meanwhile it is said that damage or cracks occurring when the particle is caused to collide against a flat plate are caused mainly by shear stress (S. Timoshenko and J. N. Goodier: "Theory of Elasticity", 2nd ed. McGRAW HILL, 1951, p. 366-372).
Accordingly, the present inventors conducted a calculation with regard to the distribution of the internal stress of the wafer on the basis of the theory of Hertz et al. (H. R. Hertz: J. Math (Crelle's J.) vol. 92, 188 and others) by assuming cases in which three types of polyhedral abrasives are made to collide vertically against the back surface of the silicon wafer. Incidentally, the calculation was made by assuming that the Poisson's ratio of silicon was 0.42 (Sze, Appendix A).
FIGS. 7A, 7B, and 7C are graphs respectively illustrating the compressive stress in the direction of the norm on their contact surfaces during collision, while FIGS. 7A', 7B', and 7C' are graphs respectively illustrating the distribution of shear stress corresponding thereto.
The depth of a point where shear stress becomes maximum is substantially equivalent to the size of the portion of contact between the wafer and the abrasive.
In each of these cases, the abrasive has a configuration in which pointed corners are provided, so that an area of contact between the wafer and the abrasive is very small. Hence, the point where shear stress becomes maximum is found in just a surface layer portion of the back surface of the wafer.
For this reason, cracks occur mainly in the surface, in which a multiplicity of flaky particulates which are liable to flake therefrom can occur.
In order to remove these flaky particulates, ultrasonic cleaning is performed after sand blasting, but the flaky particulates are not removed completely.
Some of the remaining flaky particulates flake off the wafer during the device fabrication process, reach the device region of the wafer surface, possibly causing disconnection or the like of the devices. Consequently, the yield of devices is deteriorated.
Therefore, with the conventional gettering based on sand blasting, the intensity of blasting has been reduced so as to suppress the occurrence of the flaky particulates. As a result, it has been impossible to obtain an excellent gettering effect allowing the rate of occurrence of damage to be reduced to a low level.